This study is to compare the effects of gemfibrozil & niacin combined and as single agents on levels of triglyceride-rich lipoproteins; LDL-cholesterol, and diameters and densities of LDL in individuals with endogenous lipemia and familial combined hyperlipidemia. Following control period, patients are randomized to 1 agent for 3 mos, followed by combination for 3 mos then the 2nd agent for 3 mos. The GCRC feeds some of the patients controlled diets.